2
\$\begingroup\$

I have an old telephone made in the sixties (FF-OB/ZB or FF53). Now, I was planning to make an external speaker for that telephone, but I failed miserably, so I've put that aside.

Now, the handset has a headphone, which accepts line levels and I had no problems connecting it to my sound mixer. The Microphone is a different story, though. I know it works correctly, as I can use the telephone without problems. However, when I connect it to the microphone leads (XLR connector) to the sound mixer, which has microphone amplification and phantom power (48V), the microphone appears to have a very weak pickup.

I need to turn up the gain almost to maximum, to get a signal out of it, and at that levels, it acquires huge amounts of static.

The microphone is a DKO48, here's a picture: enter image description here

I don't know, whether I should construct a preamp, or if I'm doing something fundamentally wrong. In case someone has experience with that sort of thing, please respond.

\$\endgroup\$
1
  • \$\begingroup\$ I couldn't find any data for the microphone on a Google search, beyond one thing that suggested it was a dynamic microphone. How do you have the mixer input set up? High-Z or low? Balanced or unbalanced? \$\endgroup\$
    – John R. Strohm
    Jan 28, 2012 at 4:49

1 Answer 1

Reset to default
2
\$\begingroup\$

IIRC almost all microphones used in telephones were carbon. If this is a carbon microphone (shake it and you should hear a noise of the carbon powder moving around) then you should connect it in series with a speaker and a DC power supply. That's it.

The microphone basically acts like a variable resistor.

If you want to use it to record something (that is, not just for connecting to a loudspeaker), you need to connect it in series with a resistor and a power supply, like this: schematic

To protect your equipment from DC offset, connect a small capacitor (10uF or so) in series with the "+" wire.

\$\endgroup\$
4
  • \$\begingroup\$ OK, cool, I'm gonna rig this up on a breadboard real soon. Just one thing though: It says on a datasheet here, that the DKO48 is a "Transistor microphone capsule" I don't really know what that means, though. I checked the resistance and it's at 80k ohms and dropping (the longer the multimeter was connected, the lower the resistance read). \$\endgroup\$
    – polemon
    Jan 28, 2012 at 5:06
  • \$\begingroup\$ I just checked the voltage inside the handset and, it has a DC voltage of 2.8V, so I assume you were correct! I'll check in the morning when I've rigged up the parts on the breadboard. \$\endgroup\$
    – polemon
    Jan 28, 2012 at 5:16
  • \$\begingroup\$ @polemon If this is a "transistor microphone" then you should find out which pin is positive and which is negative. It should work in the circuit provided in my answer. \$\endgroup\$
    – Pentium100
    Jan 28, 2012 at 6:00
  • \$\begingroup\$ Have my test setup rigged and it works as expected. \$\endgroup\$
    – polemon
    Jan 28, 2012 at 7:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.